Experience with gas-fired swimming pool heaters over a period of several decades has resulted in designs that accommodate the unique technical challenges involved in heating pool water. In most heaters of current design, water is passed through finned tubing through which heat from gas combustion is transferred to it. It is necessary to control the water flow rate through the heat exchanger tubes for several reasons:
1. Pool water is cool relative to the dew point of gas combustion products, making it possible to condense water vapor from those products. Unless the heater is designed to accommodate such condensation, corrosion or fouling of the heat exchanger can result. In heaters not designed for such condensation, it has been avoided by limiting the velocity of water flowing through the tubes.
2. Pool water often has high levels of dissolved solids, not only because of what may naturally occur in the water supply, but also because as water evaporates from the surface of the pool, the dissolved solids are left behind, increasing the concentration. In water heating appliances, dissolved solids tend to precipitate on the hot surfaces of heat exchangers, a phenomenon typically referred to as “liming.” The result of this precipitation is reduced heating efficiency and eventual failure of the heat exchanger. Liming can be avoided to a great extent by moving water over the heat exchanger surface at substantial velocity. In a pool heater this establishes a minimum velocity for water flow through the tubes. The required velocity depends on water temperature, however. If water is cool, lower velocities can be tolerated without liming, and conversely, higher velocities may be necessary if water is hot.
3. Flow of water in a tube can cause “erosion,” a phenomenon in which metal is removed from the tube surface by combined mechanical and chemical action. Copper, which is commonly used in swimming pool heat exchangers, is especially vulnerable to erosion. The extent of erosion depends on the water chemistry and the velocity of water flow in the tube. In the design of a pool heater the effect is to limit the velocity of water flow.
4. If water velocity is too low, the water may not be able to absorb heat at the rate the heat is delivered by the heat exchanger surface. In that case, “steam flashing” (boiling) occurs at the surface and heat transfer becomes even worse, resulting in destruction of the exchanger.
5. Some pool heaters are designed for very high heating efficiency, and in those heaters, condensation of combustion product water is intended. Extraction of heat is maximized by cooling the products to temperatures below their dew point, thereby recapturing the heat of vaporization. Heat exchanger surfaces must be as cool as possible to accomplish condensation. Several factors affect the temperature of heat exchanger surfaces, but a major factor is water velocity. High velocity cools exchanger surfaces. (Heat exchangers in high efficiency heaters typically operate with condensation only in a specific section of the exchanger and have means for handling and disposal of condensed water in that section. Water flow requirements in the condensing and non-condensing sections differ.)
6. Modern swimming pool systems often include pumping and control equipment capable of circulating pool water at differing flow rates in order to accomplish filtration and heating with minimum use of electrical energy. A common approach is to operate the circulating pump at half of normal speed when possible. On a given system, pump power changes with the cube of pump speed, so resulting energy savings are substantial. In such systems, pool heaters must be capable of operating reliably and efficiently regardless of the system flow rate.
7. At any water flow rate, heating efficiency can be increased by reducing the gas energy input. Doing so effectively increases the amount of heat transfer surface per unit of energy input. In heaters capable of operating at reduced input, control design must be done in consideration of the water flow rate.
In view of these numerous and counter-acting factors, design of a pool heater heat exchanger is a complicated process. Choice of material, geometry and the paths for water and combustion product flow are substantial elements of that process. Control of the water flow rate is equally important, since velocities are directly proportional to the flow through the exchanger.
Typically swimming pool circulation systems operate at flow rates greatly larger than necessary for pool heater heat exchangers. Therefore, most heaters include means to by-pass much of the water flow around the heat exchanger, routing only the required flow through the exchanger. In addition to providing suitable flow through the exchanger, by-pass of water reduces pressure drop through the heater, and thereby reduces pumping power.
In the past, by-pass has been accomplished with simple mechanical or thermal devices. Existing devices such as spring-loaded by-pass valves have commonly been used to pass excess water flow from the heater inlet to the heater outlet without going through the heater tubes. This is most commonly done within the heater envelope, but this by-pass can also be contained in piping outside the heater. This by-passing of water is not only important to have some control of the water flowing through the heater tubes, but also to allow a large volume of water to pass without having substantial head losses through the heater at higher water flows. In the past, to adjust for varying installation conditions, the spring within the by-pass is adjusted or changed to adjust for the pool system flow.
In another type of water flow control, a thermostatic type valve similar to a water thermostatic valve in an automobile cooling system is used to control the amount of water allowed through the heat exchanger, and to by-pass the remainder of the by-passed water to the heater outlet. Typically, the thermostatic device is used in conjunction with a spring-loaded by-pass valve.
Both the spring-loaded by-pass valve method and the thermostatic valve method have limitations on the degree of control they provide, and are also limited by a minimum flow rate at which the heater will operate.